[Remote Agents] Add gRPC transport support #18642

Implement the gRPC protocol binding for remote agents as defined in Section 10 of the A2A Specification.

Overview:
Currently, the Gemini CLI supports HTTP+JSON and JSON-RPC transports for communicating with remote agents. This task involves adding support for the GRPC transport, enabling low-latency and efficient streaming communication.

Requirements:

• Transport Integration: Enable the GRPC transport in the A2AClientManager (packages/core/src/agents/a2a-client-manager.ts).
• SDK Support: The @a2a-js/sdk already includes GRPC as a valid TransportProtocol. This task is primarily about ensuring the ClientFactory is configured to use it when an AgentCard specifies the GRPC protocol.
• Protocol Binding: Ensure the client can correctly negotiate and use the gRPC binding.
• Streaming Support: Verify that SendStreamingMessage works correctly over the gRPC transport.

Why this is straightforward:
The A2A SDK is designed with a pluggable transport architecture. Since a2a-js already provides a gRPC option, no custom transport implementation is required. It should be a matter of configuration within A2AClientManager.